The present invention concerns an inductive sensor, in particular rpm sensor.
Inductive sensors are used, for example, to detect the rpm's of a vehicle wheel as an rpm sensor for antilock systems. This is a passive inductive sensor, the function of which is based on the principle of electromagnetic induction. A ferromagnetic pulse wheel permanently mounted on the wheel axle induces a voltage that is a function of the rpm's in a coil of the sensor attached at a constant distance in that it changes the direction of the magnetic field over the rising and falling flanks of the teeth of the pulse wheel. A sensor of this type is shown in FIGS. 4 and 5. An inductive sensor of this type comprises a housing 4 in which a permanent magnet 2 having a pole pin 3 is situated. The magnet 2 comprises a ferrite core (AlNiCo) and a cold-formed pole core to conduct and bundle the magnetic lines of force. As shown in FIG. 4, the cold-formed pole core is designed partially conical in shape as a result of production. The housing 4 comprises a main housing body 5 and a coil base body 6.
As one can see in FIG. 4, the coil base body 6 comprises a conical region 6a and a cylindrical region 6b. Furthermore, coil base channels 9, 9a, 9b are situated in the coil base body 6 for accommodating wires.
To produce the coil, a wire is wound around the coil base body 6. After connection to a conductor rail by way of an input port, the wire is placed in the coil base channel 9 on the upper, larger circumference of the cone and, after a three-quarter turn, it is slid into the diagonal coil base channel, so it can then be inserted in the coil base channel on the lower, smaller, circumference of the cone (refer to FIG. 5). In the lower coil base channel, the wire is laid in place with four rotations and then directed out of the coil base channel and used to wind the coil. The area of the coil base body 6 having the smaller cylindrical diameter 6b between a first coil shield 7 and the area 6a designed in the shape of a cone is thereby filled up first until the larger diameter of the cone 6a is reached. Approximately 4200 windings out of approximately 7200 windings in all are needed in order to fill up this space at the cone 6a and the small cylindrical area 6b. The coil can then be wound in cylindrical fashion from the first coil shield 7 to the second coil shield 8.
After extrusion coating, outlines and/or cavities of the coil base channels 9, 9a and 9b are often noticeable on the outer circumference of the coil when the coil is removed. Moreover, the wire often ascends in the direction of the larger diameter of the conical area 6a when the wire is wound between the first coil shield 7 and the conical area 6a. As a result, the wire cannot be arranged on the coil in defined fashion. Additionally, this often causes snarls in the winding wire.
To protect the coil, it is covered directly with a direct extrusion coating of plastic after winding. The coil base body is usually produced out of PA6, and the extrusion coating is produced out of PA6.6. As a result, the lower melting point of PA6 (approximately 214° C.) can be used to achieve a melting-on of the ring-shaped melting tips on the coil shield in order to produce the tightness during extrusion coating with the plastic PA6.6 (melting point: approximately 228° C.) at a processing temperature at the hot nozzles of approximately 305° C.
Due to the construction described above and the manufacturing process for the inductive sensor in the related art, problems occur when the temperature changes. For example, automobile manufacturers require that the product pass a thermal shock test comprising at least 1000 temperature changes from a temperature of −30° C. to a temperature of +100° C. for 30 minutes each without fault, whereby the target temperature must be reached within five minutes. The sensors described above according to the related art only reach approximately 200 temperature changes, however. As a result, the sensors are destroyed due to fatigue fractures in the coil wire which occur, in particular, at the coil base channels and at the input port of the coil wire.
Moreover, an inductive sensor is made known in EP-632897 in which a contact zone on the ends of the windings of the coil and the conductor rails is covered with a silicone mass and a band made of polyimide material toward the injection- molded housing. Although this improves the robustness against temperature changes, the required number of 1000 temperature changes cannot be achieved using this type of inductive sensor, either.